Means and method for controlling alkylation unit to achieve and maintain a desired hydrocarbon content for recycle acid

ABSTRACT

The existing hydrocarbon content in the recycle acid in an alkylation unit is determined. Changes in earnings associated with increasing and decreasing the hydrocarbon content conditions are also determined. The interface level separating the acid phase and the hydrocarbon phase in an acid settler in the alkylation unit is controlled in accordance with the determinations to achieve and maintain a desired hydrocarbon content for the recycle acid.

United States Patent 1191 Sweeney, Jr.

MEANS AND METHOD FOR CONTROLLING ALKYLATION UNIT TO ACHIEVE AND MAINTAIN A DESIRED HYDROCARBON CONTENT FOR RECYCLE ACID Inventor: Donald E. Sweeney, Jr., New

Orleans, La. Assignee: Texaco !nc., New York, NY.

Filed: Jan. 17, 1973 Appl. No.: 324,261

DENSITY ANALYZER ACID CONTACTOR LRC DENSITY OLEFIN 8 ISOPARAFFIN SETTLER Primary Examiner-Joseph F. Ruggiero Attorney, Agent, or Firm-Thomas H. Whaley; C. G. Reis [57] ABSTRACT The existing hydrocarbon content in the recycle acid in an alkylation unit is determined. Changes in earnings associated with increasing and decreasing the hydrocarbon content conditions are also determined. The interface level separating the acid phase and the hydrocarbon phase in an acid settler in the alkylation unit is controlled in accordance with the determinations to achieve and maintain a desired hydrocarbon content for the recycle acid.

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O O m I m I mow mm W mm mvw m I1 9%: VA 512% PATENTEDJun 4:914

III IIIIIIIIIIIII II. I I

SHEET 5 DF Illllllllllllll'llllll rlllll PATENTEDJUN 41914 m& @200 E fl ll! wfimiooo MEANS AND METHOD FOR CONTROLLING ALKYLATION UNIT TO ACHIEVE AND MAINTAIN A DESIRED HYDROCARBON CONTENT FOR RECYCLE ACID BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to control systems in general and, more particularly, to a control system and method for an alkylation unit.

2. Description of the Prior Art Heretofore, systems controlling the hydrocarbon content of recycle acid in an alkylation unit were used to maintain a predetermined hydrocarbon content. Such a system is described and disclosed in US Pat. application Ser. No. 281,063, filed Aug. 16, 1973 assigned to Texaco Inc., assignee of the present invention.

The present invention distinguishes over the aforementioned in selecting a desired hydrocarbon content for the recycle acid, based on economic factors, and maintaining the hydrocarbon content at the desired value.

SUMMARY OF THE INVENTION A system controls an alkylation unit so as to achieve a desired hydrocarbon content for recycle acid in the alkylation unit. The alkylation unit includes a contactor wherein olefins and isoparaffin entering the contactor are reacted in the presence of acid. The contactor provides an acid-hydrocarbon mixture to a settler which separates the acid to provide a hydrocarbon product, including alkylate, and hydrocarbon enriched acid. A portion of the enriched acid is discharged and is replaced by fresh acid. The replenished enriched acid is recycled to the contactor. The control system controls one flow rate of fresh acid and discharge acid flow rate relatively to the other flow rate in accordance with a control signal. Characteristics of the olefin-isoparaffin charge stream, the fresh acid, the recycle acid and the hydrocarbon product are sensed by sensing circuits which provide signals corresponding to the sensed characteristics. Other devices sense the flow rates of the olefin-isoparaffincharge stream, the recycle acid, the discharge acid and the hydrocarbon product and provide corresponding signals. The interface levels between the hydrocarbon phase and the acid phase in the settler is also sensed by a circuit which provides a corresponding signal. Signals are provided which correspond to economic values related to the octane rating of the alkylate and the cost of the acid. A control signal circuit provides the control signal in accordance with the sensed characteristic signals, the sensed flow rate signals, the sensed level signal and the economic signals to provide the control signal thereby controlling the one acid flow rate so as to control the interface level to achieve the desired hydrocarbon content in the recycle acid.

The objects and advantages of the invention will appear more fully hereinafter, from a consideration of the detailed description which follows, taken together with the accompanying drawings wherein two embodiments are illustrated by way of example. It is to be expressly understood, however, that the drawings are for illustrative purposes only and are not to be construed as defining the limits of the invention.

- DESCRIPTION'OF THE DRAWINGS FIG. 1 shows a simplified block diagram of a control system, constructed in accordance with the present: invention, for controlling an alkylation unit which is also shown in partial schematic'form. FIGS. 2 through 12 are detailed block diagrams of the H computer, the programmer, the olefin signal means, the H computer, the ABA computer, the '5 computer, the G computer, the F computer. F computer, the H signal means and the set point signal means shown in FIG. 1.

FIG. 13 is a simplified block diagram of a control system, constructed in accordance with the present invention, using a general purpose digital computer.

DESCRIPTION OF THE INVENTION Referring to FIG. 1, there is shown a portion of an alkylation unit in which an olefin is reacted with isoparaffin at a predetermined temperature and in the presence of a catalyst, such as sulfuric acid and which is hereinafter referred to as the reaction acid, to form a higher molecular weight isoparaffin. For purpose of explanation, the acid in the following description shall be sulfuric acid. The olefin may be butylenes, propylene or a mixture of butylenes and propylene, while the isoparaffin may be isobutane.

The olefins and isoparaffin enter a contactor 4 by way of a line 6, where the olefins and isoparaffin are contacted with the acid entering by way of a line 7. Contactor 4 provides an acid-hydrocarbon mix by way of a line 8 to an acid settler l2. Settler l2 separates the hydrocarbon product from the acid. The hydrocarbon product, which includes alkylate, is removed through a line 14 for further processing while the acid is removed by way ofa line 16 for recycling. It should be noted that the acid from settler 12 is enriched with hydrocarbon which has not been separated by the action of settler l2. Acid settler 12 may be the only acid settler in the unit or it may be the last acid settler of a group of acid settlers.

Fresh acid enters line 16 by way of a line 17 as needed to maintain a desired recycle acid strength. A pump 20 pumps the recycle acid from line 16 into line 7. A portion of the recycle acid in line 7 is discharged by way of a line 21. The discharge-acid may be provided to another alkylation unit or disposed of.

The hydrocarbon content of the recycle acid is controlled in a manner so as to achieve a desired hydrocarbon content for the recycle acid. The interface level controls the hydrocarbon content of the recycle acid. The level of the interface of the acid phase and the hydrocarbon phase in settler '12 is controlled by controlling the fresh acid entering line 17 and the discharged acid leaving line 21. As shown, the fresh acid is controlled directly as a function of the interface level. Thus if the fresh acid flow rate is increased the acid interface level would increase since the discharge acid flow rate is not being changed. Conversely if the fresh acid flow rate is decreased, the interface level descreases. It would be obvious to one skilled in the art that the discharge acid flow rate could be controlled as a function of the interface level while maintaining the fresh acid flow rate.

The discharge acid flow rate is maintained at a predetermined rate by a flow recorder controller 24, a valve 25 and a flow rate sensor 26. The set point of flow controller 24 is set to a predetermined flow rate. Sensor 26 provides a signal E corresponding to the discharge acid flow rate R Controller 24 senses the difference between the existing flow rate and the position of the set point and controls valve 25 accordingly so that the flow rate of the discharge acid in line 21 assumes the predetermined flow rate.

A conventional type level sensor 30 detects the interface level in settler l2 and provides a signal E corresponding to the sensed level, to a level recorder controller 34. The set point in level recorder controller 34 is positioned in a manner as hereinafter described. Controller 34 provides a signal to a valve 36, for controlling the flow rate of fresh acid in line 17, in accordance with the position of the set point of level recorder controller 34 and the sensed interface level to control the interface level as heretofore described.

The hydrocarbon content of the recycle acid may be controlled so as to achieve an optimum operating condition as can be seen from the following equations:

AEA (AQHW (AA)(W where AEA is a change in earnings, AQ is the differential octane rating of the alkylate product resulting from changes in the recycle acid hydrocarbon content, W is the worth of a unit change in octane rating in dollars per octane per barrel of alkylate, AA is the differential acid consumption resulting from changes in the recycle acid hydrocarbon content in tons per barrel of alkylate, W is the cost of acid in dollars per ton.

where D D and D are the densities of the acid portion of the recycle acid, of the hydrocarbon portion of the recycle acid and of the recycle acid.

G" (1183 2 R H where a, and a are constants which by way of example may have values of 6.5 and 0068, respectively.

For the condition that P 2 P P 1.0

For the condition that P Pu, P Pn/Pl' where P is the ratio of propylene to the total olefins in the olefins-isoparaffin charge stream and P is a predetermined upper limit for the propylene-olefins ratio and may have a value of 0.6

where R is the rate of flow of olefins charged to contactor 4 in barrels per hour, R is the rate of flow of the recycle acid in barrels per hour, R is the rate offlow of the hydrocarbon product in barrels per hour, C is the internal volume of contactor 4 in barrels.

where A is the existing acid consumption in tons per barrel of alkylate; F and F are quantities calculated at the existing recycle acid hydrocarbon content H F and F are quantities calculated at any other recycle acid hydrocarbon content H li 2 b RAB/PK 11 where R is the existing discharge acid rate of flow in barrels per hour, R is the flow rate of the hydrocarbon product in barrels per hour, P, is the volume fraction of alkylate product in the hydrocarbon product, b is a conversion factor for converting barrels per hour to tons per hour and is dependent on the type of acid. By way of example, the factor b may have a value of 0.3212 for sulfuric acid.

For the condition H 2 H For the condition H H For the condition Hp 2 H For the condition H H where H is an upper limit for the recycle acid hydrocarbon content, H is a reference hydrocarbon content value for the recycle acid and may have a value of 30,

and C 1 through C are constants and by way of example for a particular alkylation unit may have values of 1.25,

1.0, 2.087406, 0.83735672 and -l0.482274, respectively.

1 afll 17.

l l fiahaaf iii.ifiiiiiiisfll where d, through d, are constants associated with a particular alkylation unit. For one such unit, d, through d,

where H and H are increased and decreased, respectively, hydrocarbon contents for the recycle acid and AH is a predetermined change for the recycle acid hydrocarbon content.

For control purposes when where H and H areupper and lower limits, respectively. for the controlled hydrocarbon content of the recycle acid.

Sensors 40, A and 40B are conventional type sensors sensing the flow rate R of the olefins and isoparaffins in line 6, the flow rate R of the recycle acid in line 7, and the flow rate R of the hydrocarbon product in line 14, respectively, and providing signals E E and E respectively.

Density analyzers 43, 43A and 438 sample the recycle acid, the fresh acid and the hydrocarbon product and provide signals E E and E respectively, corresponding to the densities D,,, D, and D to an H computer 45.

H computer provides a signal E corresponding to the hydrocarbon content H of the recycle acid in accordance with signals E E and E, and equation 3. Referring also to FlG. 2, signals E E are subtracted from signal E, by subtracting means 48 and 48A, respectively, to provide signals to a divider 49. Divider 49 divides the signal provided by subtracting means 48 with the signal from subtracting means 48A to provide a signal to a multiplier 50. Multiplier 50 multiplies the signal from divider 49 with a direct current voltage V,

6 corresponding to the term 100 in equation 3 to provide signal E Referring again to FIG. 1, signals corresponding to the ratio P of propylene to olefins and to the ratio of olefins to the total constituents in line 6 are developed as follows. Chromatograph means samples the olefins and isoparaffin stream in line 6 in response to a reset pulse E from a programmer 58. C-hromatograph means 55 in turn provides a continuous type signal E to olefin signal means and a pulse signal E to programmer 58.

Referring now to FIG. 3, the control sequence is initiated by an operator who activates an on-off" switch 63 in programmer 58 to pass a direct current voltage V to clock means 64. Clock means 64, when energized by voltage V provides a pulse E whose trailing edge triggers a one-shot multivibrator 67. The duration of time between pulses E is such as to allow the system to stabilize after each calculation. Multivibrator 67 provides reset pulse E which in turn triggers another one-shot multivibrator 68. When triggered multivibrator 68 provides a pulse E whose function will be explained hereinafter.

The pulse signal E from chromatograph means 55 is applied to a control pulse circuit 74. Control pulse circuit 74 is the same as control pulse circuit 43 described in US. application Ser. No. 257,408. Control pulse circuit 74 provides control pulses E through E Pulse E, is also provided to a one-shot multivibrator 75 which operates as a time delay. The trailing edge of the pulse provided by the triggering of multivibrator 75 triggers another one-shot multivibrator 76. Multivibrator 76, when triggered, provides a pulse E A counter in control pulse circuit 74 is reset by reset pulse E Referring now to FIG. 4, control pulses E through E, coincide with peaks of signal l5 corresponding to the different constituents of the olefin-isoparaffin stream in line 6. Each control pulse of pulses E through E J controls a corresponding sample and hold circuit of sample and hold circuits through 80l, respectively, to sample and hold a particular peak of signal E The following table relates a particular sample and hold circuit to a corresponding constituent.

Circuit Constituent 80E 80F 80G 80H 801 Normal pentane Propylene Butylenes Pcntylenes All olefinic compounds with 6 or more carbon atoms Ethane Propane lso-butanc Normal butane lso-penttine Multipliers 84 through 84! multiply the outputs from sample and hold circuits 80 through 80], respectively, with direct current voltages V through V respectively, corresponding to scaling factors pertaining to particular constituents. By way of example, voltages V through V may correspond to values of 0.02, 0.2, 1.0, 0.2, 0.15, 0.02, 0.2, 0.1, 0.02 and 0.1 volts, respectively. Sample and hold circuits 85 through 85] are con trolled by pulse E, to sample and hold the outputs from multipliers 84 through 841.

The signals from circuits 85F through 85l are summed by summing means which provides a signal corresponding to the olefins in the olefin-isoparaffin stream in line 6. A divider 93 divides the output from sample and hold circuit 85F with the signal from summing means 90 to provide a signal E corresponding to the ratio P of propylene to olefms.

Summing means 95 sums the outputs from sample and hold circuits 85 through 851 to provide a signal corresponding to the total constituents of the olefinisoparaffin stream. A divider 96 divides the signal from summing means 90 with the signal from summing means 95 to provide a signal E corresponding to the ratio of olefins to the total constituents of the stream in line 6.

Referring back to FIG. 1, signal E from olefin signal means 60 is multiplied with olefin-isoparaffin flow rate signal E by a multiplier 100 to provide a signal E corresponding to the flow rate R of olefins in line 6.

A signal E corresponding to the percent volume P of alkylate in the hydrocarbon product is derived in a manner similar to the generation of signal E Chromatograph means 55A periodically samples the hydrocarbon product in response to reset pulse E Chromatograph means 55A provides signal E to alkylate signal means 105 and a pulse signal E to programmer 58.

Referring again to FIG. 3, pulse signal E is applied to a control pulse circuit 74A which in turn provides control pulses E through E One shot multivibrators 75A and 76A are responsive to control pulse E to provide control pulse E Referring to FIGS. 1 and 4, control pulses E through E are provided to alkylate signal means 105 which is similar to olefin signal means 60. It can be appreciated that a repetition of the details of signal means 60 would be cumbersome. It is sufficient to say that alkylate signal means 105 differs from olefin means 60 in that summing means 90 and divider 93 are not present in alkylate signal means 105. Alkylate signal means 105 further differs in that the output from sample and hold circuit 85L corresponding to the constituent having 6 or more carbon atoms, is applied to divider 96 in place of the output from the deleted summing means 90 so that divider 96 in alkylate signal means 105 provides signal E Referring now to FIGS. 1 and 5, H(' computer 110 receives signal E, from H computer 45 and provides signals Em. E corresponding to the terms H and H respectively in the aforementioned equations. Signal E is applied to summing means 112 and to subtracting means 113 which also receive a direct current voltage V corresponding to the predetermined change AH in the hydrocarbon content of the recycle acid. Summing means 112 adds voltage V and signal E to provide a signal corresponding to an increased hydrocarbon content while subtracting means 113 subtracts voltage V from signal E to provide a signal corresponding to a decreased hydrocarbon content. The signals from means 112 and 113 are applied to comparators 114, 114A along with direct current voltages V and V respectively. Voltages V V,; correspond to an upper limit and a lower limit, H and H respectively, for the recycle acid hydrocarbon content.

Comparator 114 provides a high level direct current output when the signal from summing means 112 is not more positive than voltage V and a low level output when the signal from summing means 112 is more positive than voltage V The output from comparator 114 controls an electronic switch receiving the signal from summing means 112 and signal E When the increased hydrocarbon content signal exceeds the upper limit for the hydrocarbon content, the low level output from comparator 114 causes electronic switch 116 to pass signal E, as the H signal E When the increased hydrocarbon content signal is not more positive than voltage V switch 116 is controlled to pass the signal from summing means 112 as the H signal E Comparator 114A and switch 116A cooperate in a similar manner. using the signal from subtracting means 113 and voltage V... Comparator 114A provides a high level direct current output when voltage V is more negative than signal E and a low level output when voltage V is not more negative than signal E Switch 116A passes the signal from subtracting means 113 as a signal E when comparator 114A provides a low level output and signal E as signal E when comparator 114A provides a high level output.

Changes in earnings AEA and AEA for an increased hydrocarbon content and for a decreased hydrocarbon content. respectively, are computed. A target hydrocarbon content is determined from the computation. In FIG. 1, signals E E E E E E and E are applied to computers 130, 130A. Computer 130 provides a signal E corresponding to the change AEA in earnings resulting from an increased hydrocarbon content in accordance with the received signals and signal E from.H computer and equation 1.

Referring now to FIG. 6, a comparator 133 in computer compares signal E with a direct current voltage V, corresponding to an upper limit H for the ratio of propylene to olefins. When voltage V is more negative than signal E comparator 133 provides a high level direct current output to an electronic switch 134, when voltage V, is not more negative than signal E the output from comparator 133 goes to a low level. Signal E voltage V are also applied to a divider which divides signal E with voltage V, to provide a signal corresponding to the ratio of P /P The signal from divider 135 is applied to electronic switch 134 which also receives a direct current voltage V corresponding to 1.0. Electronic switch 134 passesthe signal from divider 135 when the output from comparator 133 is at a low level as a signal E and voltage V as signal E when the output from comparator 133 is at a high level. Signal B is provided to G computers 138, 138A which also receives signals E and E respectively.

Signals E E and E are applied to S computers 140, 140A. S computer 140 receives signal E while S computer 140A receives signal E S computers 140, 140A provide signals E and E respectively, corresponding to the terms S and S respectively, to G computers 138 and 138A, respectively. Referring to FIG. 7, signals E E, are summed by summing means 142, in computer 140, to provide a signal corresponding to the term R -l-R A multiplier 143 multiplies the signal from 142 with signal E to provide a signal to a multiplier 144. Multiplier 144 multiplies the signal from multiplier 143 with voltage V to provide a signal corresponding to the numerator in equation 8. Subtracting means 148 subtracts signal E from voltage V to provide a signal corresponding to the term 100-H to a multiplier 149 where the signal from subtracting means 148 is multiplied with signal E. A multiplier I50 multiplies the signal from multiplier 149 with the direct current voltage V corresponding to the term C to provide a signal corresponding to denominator of equation 8. A divider 151 divides the signal from multiplier 144 with the signal from multiplier 150 to provide signal E23,

Computer 140A is similar to computer 140 except that signal E is used in lieu of signal E to provide signal E Referring now to FIG. 8, a multiplier 158 in G computer 138 squares signal E to provide a signal corresponding to the term S in equation 5. A multiplier 159 multiplies the signal from multiplier 158 with a direct current voltage V corresponding to the term a, in equation 5, to provide a signal to summing means 160. A multiplier 163 multiplies signals E E together to provide a signal corresponding to P H to another multiplier 164. Multiplier 164 multiplies the signal from multiplier 163 with a direct current voltage V corresponding to the term a Summing means 160 sums the outputs from multipliers 159, 164 to provide a signal E corresponding to the quantity G G computer 138A is similar to computer 138 except that computer 138A provides signal E instead of signal E and uses signal 15, in lieu of signal E Referring again to FIG. 6, subtracting means 170 subtracts signal E from signal E to provide a signal E corresponding to the term A0. A multiplier 171 multiplies signal E with a direct current voltage V corresponding to the economic value W in equation 1 to provide a signal E F computers 175 and 175A receive signals E E and E E respectively, to provide signals E and E respectively. Signals E and E correspond to the terms F and F in equation 10.

Referring now to FIG. 9, signal E is squared by multiplier 177 to provide a signal corresponding to the term 5,3. A voltage V corresponding to the mathematical constant eis applied to a conventional type logarithmic amplifier 178 which provides a signal corresponding to log e. A multiplier 179 multiplies the signal from multiplier 177 with the signal from amplifier 178 to provide a signal corresponding to the term S log e. Subtracting means 184 subtracts signal E from voltage V to provide a signal corresponding to the term (lP,,) in the equation 16. A divider 185 divides signal E with the signal from subtracting means 184 to provide a signal corresponding to the term P,,/(l-P,,). Multipliers 186, 187 and 188 cooperate to raise the signal from multiplier 185 to powers of 2, 3 and 4 so that multipliers 186, 187, 188 provide signals corresponding to the terms [P l-PB), [Pfl] l Pn) and [P,,]/( lP,,), respectively. Multipliers 190 through 193 multiply the signals from divider 185 and multipliers 186, 187, 188 with direct current voltages V V V and V respectively, corresponding to the terms d,, d d and d respectively, in equation 16. Summing means 200 sums the outputs from multipliers 190 through 193. A multiplier 201 multiplies the output from summing means 200 with the signal from multiplier 179 to provide a signal to an antilog circuit comprising an operation amplifier 202 with a function generator 203 as a feedback element. Function generator 203 may be of the type manufactured by Electronic Associates as their part number PC-12. Operational amplifier 2112 provides signal E corresponding to the term F Referring again to FIG. 6, F computer 175A is similar to computer 175 except that signal E is used in lieu of signal E A divider 210 divides signal E with signal E to provide a signal E corresponding to the term F /F An F computer 215 receives signal E and provides a signal E corresponding to one value for the term F Referring also to FIG. 10, there is shown- F H computer 215 which includes subtracting means 217 subtracting a direct current voltage V,,,, corresponding to a reference hydrocarbon content H from signal E to provide a signal corresponding to the term Pi -H A divider divides the signal from subtracting means 217 with voltage V, to provide a signal corresponding to the term (H H )/l00. Multipliers 220 and 221 raise the last mentioned term to the powers of 2 and 3. Multipliers 222, 223 and 224 multiply the signals from divider 218 and multipliers 220 and 221, respectively, with direct current voltage V V and V respectively. Voltage V V and V correspond to terms C C and C respectively, in Equation 13. Summing means 225 sums the signals from multipliers 222, 223 and 224 with a direct current voltage V corresponding to the constant C to provide signal E F computer 215A is similar to computer 215 except that signal E is used in place of signal E Computer 215A provides signal E corresponding to one value for the F As indicated in the equations, the one value for F as represented by signal E 2, is used if a certain condition exists. The necessary condition is that hydrocarbon content H must be less than an upper limit H for the hydrocarbon content. Referring back to FIG. 6, a comparator 230 compares signal E, with a direct current voltage V corresponding to H to provide an output to electronic switch 231. Comparator 230 provides a low level output when signal E is not more negative than voltage V and a high level output when signal E is more negative than voltage V Electronic switch 231 receives signal E and a direct current voltage V corresponding to the term C When comparator 230provides a high level output, switch 231 passes signal E as signal E corresponding to the term F,,,,. When the hydrocarbon content H is not less than the upper limit hydrocarbon content H comparator 230 provides a low level output causing electronic switch 231 to pass voltage V as signal B In a similar manner, comparator 230A and electronic switch 231A cooperate to compare signal E corresponding to the hydrocarbon content H with voltage V corresponding to H so that signal E corresponding to the one value of F is passed as signal E when the hydrocarbon content is less than the upper limit H and voltage V corresponding to the term C is passed as signal E when the hydrocarbon content H is not less than the upper limit H A divider 235 divides signal E with signal E to provide a signal E corresponding to the term F F A multiplier 236 multiplies signals E E;,,, to provide a signal to subtracting means 237. Subtracting means 237 subtracts voltage V,, from multiplier 236 to provide a signal corresponding to the term scF SH HC)- A multiplier 240 multiplies signals E E to provide a signal corresponding to the term R P Another multiplier 241 multiplies signal E with a'direct current voltage V corresponding to the constant b to provide a signal corresponding to the term bR, to divider 242. Divider 242 divides the signal from multiplier 241 with the signal from multiplier 240 to provide a signal corresponding to the acid consumption A A multiplier 243 multiplies the output from subtracting means 237 with the signal from divider 242 to provide a signal E corresponding to the term AA. A multiplier 244 multiv plies signal E with a direct current voltage V corresponding to the term W to provide a signal E Subtracting means 250 subtracts signal E from signal E to provide signal E AEA computer 130A operates in a similar manner to provide signal E the difference being that signal E is used in computer 130A wherever signal E is used in computer 130 so that signal E corresponds to the change in earnings'to a decrease in hydrocarbon content.

Referring to FIGS. 1 and 11, signals E E from computers 130 and 130A, respectively, are applied to target hydrocarbon content H signal means 251, along with signals E E and E H signal means 251 is essentially a selective switch for passing the H H or H signal as the target recycle acid hydrocarbon content signal E In signal means 251, comparators 252 and 252A compare signals E and E respectively, with a zero reference potential. such as ground. While another comparator 252B compares signals E E with each other. For the condition that the AEA earning is positive and is more positive than the AEA signal means 251 provides the H signal E as the H signal E For this condition, comparators 252 and 2528 provide high level outputs which enable an AND gate 253 causing AND gate 253 to provide a high level output. The high level output from AND gate 253 renders an electronic switch 254 conductive to pass signal E -as signal E The output from AND gate 253 is inverted to a low level output by an inverter 255 which disables another AND gate 256. When disabled, AND gate 256 provides a low level output which renders an electronic switch 254A receiving signal E non-conductive, thereby blocking signal E The high level output from comparator 252B is inverted to a low level output by an inverter 257 causing another AND gate 258 to provide a low level output to another elec tronic switch 2548. Switch 2548 is rendered nonconduetive by the low level output and blocks signal zo- For the condition that signal E is positive and is more positive than signal E comparators 252A and 2528 provide a high level and a low level output, respectively. Inverter 257 inverts the output from comparator 2528 to a high level so that AND gate 258 receiving two high level signals provides a high level output to switch 254B. Switch 2548 passes signal E as signal E in response to the high level output from AND gate 258. The output from comparator 2528 causes AND gate 253 to provide a low level output to switch 254 causing switch 254 to block signal E The high level output from AND gate 258 is inverted to a low level by an inverter 259 which causes AND gate 256 to provide a low level output to switch 254A. Switch 254A blocks signal E in response to the low level output from AND gate 256.

' AND gates 253 and 258, respectively, causing them to provide low level outputs. Inverters 255, 259 invert the signal E by a low level output from AND gate 280 in low level outputs to high levels causing AND gate 256 to provide a high level output rendering switch 254A conductive. When rendered conductive, switch 254A passes signal E as signal E The low level outputs from AND gates 253, 258 render switches 254 and 2548, respectively, non-conductive to block signals E and E respectively.

Referring now to FIGS. 1, 3 and 12, H signal means 251 provides signal E to set point signal means 270 which also receives signals E E E and E Set point signal means 270 provides a signal E for adjusting the set point of level recorder controller 34. Subtracting means 271 subtracts signal E from signal E to provide a signal corresponding to the expression (H -H in the following equation:

ALD B T) where AL,, is a calculated desired change in the interface level of settler 12 due to the new target hydrocarbon content and k is a constant. By way of example k may have a value of 0.5.

A multiplier 272 multiplies the signal from subtracting means 271 with a direct current voltage V corresponding to the term k in equation 22, to provide a signal E which corresponds to the desired change in interface level AL when the target hydrocarbon content is changed. However, there are limitations on the interface level change allowed during one cycle of operation. A selector circuit 273 receives signal E from multiplier 272 and direct current voltages V V corresponding to an upper limit change AL and a lower limit change AL for the interface levelv Selector circuit 273 includes comparators 275 and 275A which compare the signal E with voltages V;;, and V respectively. I

For the condition that the desired change AL,, is within the limits, comparators 275 and 275A provide high level outputs causing an AND gate 280 to provide a high level output. The high level output from AND gate 280 renders an electronic switch 281 conductive to pass signal E as signal E the selected change in interface level AL The high level outputs from comparators 275, 275A are inverted to low levels by the inverters 283, 283A to render electronic switches 281A and 2818, respectively, non-conductive to block voltages V and V respectively.

For the conditions that AL exceeds the upper limit AL comparators 275 and 275A provide a low level output and high level output, respectively. Inverter 283 inverts the low level output from comparator 275 to a high level to enable switch 281A to pass voltage V as AL signal E AND gate 280 provides a low level output in response to the low level output from comparator 275 to render switch 281 non-conductive thereby blocking signal E The high level output from comparator 275A is inverted to a low level by inverter 283A to render switch 281B non-conductive thereby blocking voltage V For the condition that the interface level change'AL exceeds the lower limit AL comparators 275, 275A provide a high level and a low level output, respectively. Switch 281 is rendered non-conductive to block response to the low level output from comparator 275A. The high level output from comparator 275 is inverted to a low level by inverter 283 to render switch 281A non-conductive thereby blocking voltage V;,,. The output from comparator'275A is converted to a high level by inverter 283A rendering switch 2818 conductive to pass voltage V as AL to signal E Summing means 285 sums signal E and E to provide a signal E corresponding to a calculated desired interface level L,,. Selector circuit 273A, which is similar to circuit 273, compares signal E,,, with direct current voltages V and V corresponding to an upper limit L and a lower limit L respectively, for the interface level in settler l2. Selector circuit 273A provides signal E as the selected interface level L signal E when the calculated interface level L is within the limits defined by L,' and L Selector circuit 273A provides voltage V as signal E when the calculated interface level L exceeds the upper limit L, and provides voltage V,,, as signal E,, when the calculated interface level L exceeds the lower limit L Signal E is applied to a sample and hold circuit 289 and to an electronic switch 290. Pulse E,,, from programmer 58 causes sample and hold circuit 289 to sample and hold signal E Sample and hold circuit 289 prevents calculations for a new value of signal E from adversely affecting the alkylation unit until such time as the calculation is complete.

Signal E normally passes through electronic switch 290 to become signal E which positions the set point of level recorder controller 34. However, during calculations, switch 290 passes the output from sample and hold circuit 289 as signal E in response to pulse E,,.

It can be determined from FIG. 3 that pulse E occurs before pulse 14. The pulse E, is of suitable width so as to terminate after the calculations.

The system of the present invention-determines adesirable value for the hydrocarbon content for the recycle acid in an alkylation unit and controls the hydrocarbon content of the recycle acid to maintain it at the de- 1 sirable value.

Although the present invention is shown using analog computers, it would be obvious to one skilled in the art that signals E, through E,,, E and E, may be converted to digital signals and a general purpose digital computer may be programmed to provide the control function as taught by the present invention. Naturally, the output from the digital computer may be converted to analog signal such as signal E,,.

What is claimed is:

l. A system for controlling an alkylation unit to achieve a desired hydrocarbon content in recycle acid, said alkylation unit includes a contactor wherein olefinsisoparaffin entering the contactor are contacted in the presence of acid and said contactor provides an acidhydrocarbon mixture to a settler which separates the acid to provide a hydrocarbon product, including alkylate, and hydrocarbon enriched acid, a portion of the enriched acid is discharged, fresh acid is added to the remaining enriched acid which is then provided to the contactor as recycle acid, comprising means for controlling one flow rate of the fresh acid and discharge acid flow rates relative to the other flow rate in accordance with a control signal; means for sensing characteristics of the olefins-isoparaffin, the fresh acid, the recycle acid and the hydrocarbon product and providing signals corresponding thereto; means for sensing the flow rates of the olefins-isoparaffin, the recycle acid, the discharge acid and the hydrocarbon product and 14 providing corresponding signals; means for sensing the interface level in the settler and providing a signal representative thereof; means for providing signals W and W corresponding to economic values related to the octane rating of the alkylate and to the cost of the acid, respectively; and means connected to all of the other means for providing the control signal to the control means in accordance with the characteristic signals, the flow rate signals, the level signal and the economic signals to control the one acid flow rate so as to control the interface level to achieve the desired hydrocarbon content in the recycle acid, and said control signal means includes means connected to the characteristics sensing means for providing a signal corresponding to the hydrocarbon content H,, of the recycle acid in accordance with the characteristic signals; means connected to the H signal means for providing signals corresponding to an increased hydrocarbon content Hp for the recycle acid and for a decreased hydrocarbon content H for the recycle acid; means connected to the H signal means and to the H H signal means for providing signals corresponding to a change in earnings AEA, for an increased hydrocarbon content condition and AEA for a decreased hydrocarbon content condition in accordance with the H H and H signals, and means connected to the H signal means, to the H -,H signal means and to the earnings change signal means for selecting the H,,, H or H signal in accordance with the ABA, and AEA, signals as a desired hydrocarbon content condition and for providing a signal corresponding thereto as the control signal.

2. A system as described in claim 1 in which the earnings change signal means includes AQ signal means for providing signals corresponding to a change in octane rating A0, for the increased hydrocarbon condition and to a change in octane rating A0 for the decreased hydrocarbon condition, AA signal means for providing signals corresponding to a change in acid consumption AA, for the increased hydrocarbon content condition and to a change in acid consumption AA, for the decreased hydrocarbon condition, and means connected to the economic value signal means, to the octane change signal means and to the acid consumption change signal means for providing the ABA, and AEA signals in accordance with the'following equation:

where the ABA, signal is derived by using the A0, and AA, signals, where A0, and AA, are substituted for A0 and AA, respectively, and the AEA signal is derived by using the A0 and AA, signals, where A0 and AA replace A0 and AA, respectively.

3. A system as described in claim 2 in which the AO signal means includes G signal means for providing signals corresponding to a quantity G for the increased hydrocarbon content condition, to a quantity G for a decreased hydrocarbon content condition and to a quantity 6,, for the existing hydrocarbon content condition, and means connected to the G signal means for providing the AQ, and A02 signals in accordance with the G G and 0,, signals and the following equation:

where the AQ signal is developed by using the G and G signals, where G is substituted for G and the AQ, is generated by using the G and G signals, where G is substituted for G in the last mentioned equation.

4. A system as described in claim 3 in which the AA signal means includes means for providing a signal corresponding to the existing acid consumption A,,, F, signal means for providing signals corresponding to the quantities F F and F for the existing hydrocarbon content condition, the increased hydrocarbon content condition, and the decreased hydrocarbon content condition, respectively, F signal means for providing signals corresponding to the quantities F,,,,, F,,,;, and F for the existing hydrocarbon content condition, the increased hydrocarbon content condition, and the decreased hydrocarbon content condition, respectively, and means connected to the acid consumption signal means, to the F signal means, to the F signal means for providing signals corresponding to AA, and to AA, in accordance with the A F F F and F signals and the following equation:

where the AA, signal is generated using the P and F signals, where F and F "('1 are substituted for P and F respectively, and the AA, signal is generated using the Fspz and FHCZ signals, where F502 and Ff? replace F and F respectively.

5. A system as described in claim 4 in which the A,, signal means includes means for providing a signal corresponding to a constant b, means connected to the characteristic sensing means for providing a signal corresponding to the volume fraction P of alkylate in the hydrocarbon product, and means connected to the flow rate sensing means for providing the A signal in accordance with the following equation:

where R is the other flow rate of the fresh acid and discharge acid flow rates and R is the flow rate of the hydrocarbon product.

6. A system as described in claim 5 in which the G signal means includes means for providing signals cor responding to constants a, and a means for providing a signal corresponding to a quantity P,,; S signal means for providing signals corresponding to quantities S S and S for the existing hydrocarbon content condition, the increased hydrocarbon content condition and the decreased hydrocarbon content condition, respectively, and means connected to the a,-a P,,, S, H H -H signal means for providing the G C1,, and 0,, signals in accordance with the (1,, (1,, P,,, S 8,, 5,, H,,, H,-, and H signals and the following equation:

where the 0,, signal is developed using the S and H signals, where 8,, and H are substituted for S and H, respectively, the 6,, signal is developed using the S, and H signals, where S, and H,-, are substituted for S and H, respectively, and the G signal is developed using the S and H02 signals, where S, and H are substituted for S and H, respectively.

7. A system as described in claim 6 in which one of the signals provided by the characteristic sensing means corresponds to the ratio P ofpropylene to olefins; and the P signals means includes means for providing a signal corresponding to an upper limit P, for the ratio of propylene to olefms, means connected to the characteristic sensing means and to the P,' signal for comparing the P and P,,- signals, means connected to the characteristic signal means and to the P signal means for providing a signal corresponding to P /P means for providing a signal corresponding to 1.0, and

switching means controlled by the comparing means to pass the 1.0 signal as the P signal while blocking the P /P signal when the ratio P is equal to or greater than the limit ratio P and to pass the P /P signal as the P signal while blocking the 1.0 signal when the ratio P is less than the limit ratio P 8. A system as described in claim 7 in which the S signal means includes means for providing a signal corresponding to the volume C, of the contactor, means connected to the characteristic sensing means and to the flow rate sensing means for providing a signal corresponding to the flow rate R of the olefms in accordance with the characteristic signals and the olefinisoparaffin flow rate signal, means connected to the C signal means, to the R signal means, to the H signal means, to the flow rate sensing means, and to the H,-,H signal means for providing the S S, and S signals in accordance with the R,,, C,.-, H,,, H and H signals, the flow rate signals and the following equation:

where R,, and R H are the flow rates of the recycle acid and hydrocarbon product, respectively, the S S, and S signals are developed using the H H,-, and H respectively, where S,,, S, and S are substituted for H in the last mentioned equation.

9. A system as described in claim 8 further comprising means for providing a signal corresponding to the upper limit H,,, for the hydrocarbon content; and in which the F signal means includes means connected to the H signal means and to H signal means for comparing the H signal with the H signal, means for providing signals corresponding to constants C, through C means for providing a signal corresponding to a reference hydrocarbon content H for the recycle acid, means connected to the C ,C signal means, to the H signal means, to the H,',-H signal means and to H signal means for providing signals corresponding to F,,,,,, F and F in accordance with the C through C signals and the H H,-,, H and H signals and the following equation:

where the F F and F' signals are derived using the H H,-, and H signals where F,,,,, F and F are substituted for H, respectively, in the last mentioned equation, and switching means connected to the last mentioned comparing means, to the C,C signal means and to F, F,,,-, and F signal means and controlled by the last mentioned comparing means to provide the C, signal as the F F and F signals while blocking the F',,,,, P and F signals when the existing hydrocarbon content H is not less than the upper limit H to the hydrocarbon content and to provide the FIN, F'H(-1 and F ygg Signals as the FUR, F116] and P signals, respectively, while blocking the C,

signal when the existing hydrocarbon content H is less than the upper limit l-l hydrocarbon content.

. 10. A system as described in claim 9 in which the F,

where the F,,,,, F and F signals are obtained by using S,,, S, and S signals, respectively, where S,,, S, and S are substituted for S in the last mentioned equation.

11. A system as described in claim 1 in which the control signal means is a digital computer programmed in a predetermined manner to provide the control signals.

12. A system as described in claim 11 in which the computer has been programmed to determine changes AEA, and AEA in earnings for an increased hydrocarbon content H for the recycle acid and a decreased hydrocarbon content H for the recycle acid in accordance with the following equations:

where AQ, and AQ- are changes in the octane ratings for the increased and decreased hydrocarbon content conditions, respectively, AA, and AA are changes in the acid consumption for the increased and decreased hydrocarbon content conditions, respectively, and W and W, are economic values associated with the octane rating and the acid.

13. A method for controlling an alkylation unit to achieve a desired hydrocarbon content in recycle acid, said alkylation unit includes a contactor wherein olefins-isoparaffin entering the contactor are contacted in the presence of acid and said contactor provides an acid-hydrocarbon mixture to a settler which separates the acid to provide a hydrocarbon product, including alkylate, and hydrocarbon enriched acid, a portion of the enriched acid is discharged, fresh acid is added to the remaining enriched acid which is then provided to the contactor as recycle acid, which comprises controlling one flow rate of the fresh acid and discharge acid flow rates relative to the other flow rate in accordance with a control signal; sensing characteristics of the olefins-isoparaffin, the fresh acid, the recycle acid and the hydrocarbon product; providing signals corresponding to the sensed characteristics; sensing the flow rates of the olefins-isoparaffin, the recycle acid, the discharge acid and the hydrocarbon product; providing signals corresponding to the sensed flow rates; sensing the interface level in the settler; providing a signal represent'ative of the sensed interface level; providing signals W and W,, corresponding to economic values related to the octane rating of the alkylate and to the cost of the acid, respectively; and providing the control signal in accordance with the characteristic signals, the flow rate signals, the level signal and the economic signals to control the one acid flow rate so as to control the interface level to achieve the desired hydrocarbon content in the recycle acid and said control signal step includes providing a signal corresponding to the hydrocarbon content H,, of the recycle acid in accordance with the characteristic signals; providing signals corresponding to an increasedhydroearbon content Hg for the recycle acid and for a decreased hydrocarbon content H for the recycle acid; providing signals corresponding to a change in earningsAEA, for an increased hydrocarbon content condition and AEA for a decreased hydrocarbon content condition in accordance with the H H and H signals, selecting the H H, or H signal in accordance with the ABA, and AEA, signals as a desired hydrocarbon content condition, and providing a signal corresponding to the selected signal as the control signal. I

14. A method as described in claim 13 in which the earnings change signal step includes providing signals corresponding to a change in octane rating A0, for the increased hydrocarbon condition and to a change in octane rating A0 for the decreased hydrocarbon condition, providing signals corresponding to a change in acid consumption AA, for the increased hydrocarbon content condition and to a change in acid consumption AA for the decreased hydrocarbon condition, and providing the ABA, and AEA signals in accordance with the following equation:

AEA (AQ) (W (WA) where the ABA, signal is derived by using the A0, and AA, signals, where A0, and AA, are substituted for AQ and AA,, respectively, and the ABA, signal is derived by using the A0 and AA, signals, where A0 and AA dance with the G G and G signals and the following equation:

AQ GC GB where the A0, signal is developed using the G and G signals, where G is substituted for G and the AQZ signal is developed using the G and G signals, where G is substituted for G in the last mentioned equation.

16. A method as described in claim 15 in which the step of providing the AA, and AA signals includes providing a signal corresponding to the existing acid consumption A providing signals corresponding to the quantities F,,-,,, F and F, for the existing hydrocarbon content condition, the increased hydrocarbon content condition and the decreased hydrocarbon content condition respectively, providing signals corresponding to the quantities F F and F, for the existing hydrocarbon content condition, and for the decreased hydrocarbon content condition, respectively, providing the signals corresponding to AA, and to AA, in accordance with the A F F,,,,Fsn and F signals and the following equation:

where the AA, signal is provided using the F and F signals where F and F are substituted for F and F respectively, and the AA; signal is developed using the P,,- and F signals where P,,- and G are substituted for F and F respectively, in the last mentioned equation.

17. A method as described in claim 16 in which the step of providing the A,, signal includes providing a signal corresponding to a constant b, providing a signal corresponding to the volume fraction P,, of alkylate in the hydrocarbon product, and the A signal in accordance with the following equation:

where R is the other flow rate of the fresh acid and discharge acid flow rates and R is the flow rate of the hydrocarbon product.

18. A method as described in claim 17 in which the step of providing the G,-,, G and (3,, signals includes providing signals corresponding to constants a, and providing a signal corresponding to a quantity P,,; providing signals corresponding to quantities S S, and S for the existing hydrocarbon content condition, the increased hydrocarbon content condition and the decreased hydrocarbon content condition, respectively; and providing the G,,, G, and G signals in accordance with the a,, a P,,, S,,, 8,, 8,, H,,, H and H signals and the following equation:

where the (1,, signal is developed using the 5,, and H signals, where S,, and H are substituted for S and H, respectively, the G,-, signal is developed using the S, and H,-, signals where S, and H('1 are substituted for S and H, respectively, and the G signal is developed using the S and H signals, where S and H are substituted for S and H, respectively, in the last mentioned equation.

19. A method as described in claim 18 in which one of the characteristic signals provided corresponds to the ratio P,, of propylene to olefins; and the step of providing the P,, signal includes providing a signal corresponding to an upper limit P,- for the ratio of propylene to olefins, comparing the P,, and P, signals, providing a signal corresponding to P,,/P,, providing a signal corresponding to 1.0, and providing the LO signal as the P,, signal when the ratio P,, is equal to or greater than the limit ratio P, and providing the p,,/P,- signal as the P,, signal when the ratio P,, is less that the limit ratio P,,.

20. A method as described in claim 19 in which the step of providing the S signal includes providing a signal corresponding to the volume C, of the contactor, means connected to the characteristic sensing means and to the flow rate sensing means for providing a signal corresponding to the flow rate R of the olefins in accordance with the characteristic signals and the where R and R are the flow rates of the recycle acid and hydrocarbon product, respectively, the S,,, S, and S signals are developed using the H,,, and H signals, respectively, where l-l,,, H and H are substi tuted for H in the last mentioned equation.

21. A method as described in claim 20 further comprising the step of providing a signal corresponding to an upper limit H for the hydrocarboncontent; and in which the step of providing the F signal includes comparing the H signal with the H signal, providing signals corresponding to constants C, through C providing a signal corresponding to a reference hydrocarbon content H for the recycle acid, providing the F,,,,-, F and F signals in accordance with the C through C signals and the H,,, H,-,, H and H signals and the following equation:

where the F,,,,,, F and F signals are derived using the H,,, H and H signals, respectively, where H,,, H and H are substituted for H in the last mentioned equation, providing the C, signal as the F,,,,, F,,,-, and F signals when the existing hydrocarbon content H H is not less than the upper limit H hydrocarbon content and providing the F,,,,, F,,,-, and F signals as the F,,,,, F,,,, and F signals, respectively, when the existing hydrocarbons content H,, is less than the upper limit H hydrocarbon content.

22. A method as described in claim 21 in which the step of providing the P,, signal includes providing signals corresponding to constants d, through (1,, and providing the signals corresponding to the quantities F F and F in accordance with the S,,, S,, S d, through d, and P,, signals and the following equation:

equation.

w talll gz gy UNITED STATES PATENT @FFICE {TZ'ER'IIFEA'EE 0F WI'MON Pmfiemt 1%,, 3 y 9 Da ted June 14 9 7 Iwwmflg} Donald E. Sweeney, Jr.

It is certified that error appears in the above-"identified patent end that said Lettere Patent are her e by eqrr eet ged as shown below:

* column 17, line 31: igEA QAQi) (w .1 A (WA) shoeld read V --AEA AQ w AA (W Column 17, line 32: "A EA2=(AQZMWQH)Q=(XZ )'(YJA) tsholuld read "--A EA '='(AQ )(W)-(AA (Ywlumn 18, line 29 "AEA=(AQ) (w )=(A A) W should read wAEA-- A@ w A A) w Signed and sealed this 5th day of November 1974.

(SEAL) V ATIEQSIH Meow M0 GIBSN JR. 0. MARSljALL DANN Attestime; @ffieer Commissioner of Patents 

2. A system as described in claim 1 in which the earnings change signal means includes Delta Q signal means for providing signals corresponding to a change in octane rating Delta Q1 for the increased hydrocarbon condition and to a change in octane rating Delta Q2 for the decreased hydrocarbon condition, Delta A signal means for providing signals corresponding to a change in acid consumption Delta A1 for the increased hydrocarbon content condition and to a change in acid consumption Delta A2 for the decreased hydrocarbon condition, and means connected to the economic value signal means, to the octane change signal means and to the acid consumption change signal means for providing the Delta EA1 and Delta EA2 signals in accordance with the following equation: Delta EA ( Delta Q)(WQ) - ( Delta A)(WA) where the Delta EA1 signal is derived by using the Delta Q1 and Delta A1 signals, where Delta Q1 and Delta A1 are substituted for Delta Q and Delta A, respectively, and the Delta EA2 signal is derived by using the Delta Q2 and Delta A2 signals, where Delta Q2 and Delta A2 replace Delta Q and Delta A, respectively.
 3. A system as described in claim 2 in which the Delta Q signal means includes G signal means for providing signals corresponding to a quantity GC1 for the increased hydrocarbon content condition, to a quantity GC2 for a decreased hydrocarbon content condition and to a quantity GB for the existing hydrocarbon content condition, and means connected to the G signal means for providing the Delta Q1 and Delta Q2 signals in accordance with the GC1GC2 and GB signals and the following equation: Delta Q GC-GB where the Delta Q1 signal is developed by using the GC1 and GB signals, where GC1 is substituted for GC and the Delta Q2 is generated by using the GC2 and GB signals, where GC2 is substituted for GC in the last mentioned equation.
 4. A system as described in claim 3 in which the Delta A signal means includes means for providing a signal corresponding to the existing acid consumption AB, FS signal means for providing signals corresponding to the quantities FSB, FSC1 and FSC2 for the existing hydrocarbon content condition, the increased hydrocarbon content condition, and the decreased hydrocarbon content condition, respectively, FH signal means for providing signals corresponding to the quantities FHB, FHC1 and FHC2 for the existing hydrocarbon content condition, the increased hydrocarbon content condition, and the decreased hydrocarbon content condition, respectively, and means connected to the acid consumption signal means, to the FS signal means, to the FH signal means for providing signals corresponding to Delta A1 and to Delta A2 in accordance with the ABFSC, FHB, FSB and FHC signals and the following equation: Delta A AB((FSCFNB/FSBFHC) - 1) where the Delta A1 signal is generated using the FSC1 and FHC1 signals, where FSC1 and FHC1 are substituted for FSC and FHCrespectively, and the Delta A2 signal is generated using the FSC2 and FHC2 signals, where FSC2 and FHC2 replace FSC and FHC, respectively.
 5. A system as described in claim 4 in which the AB signal means includes means for providing a signal corresponding to a constant b, means connected to the characteristic sensing means for providing a signal corresponding to the volume fraction Pk of alkylate in the hydrocarbon product, and means connected to the flow rate sensing means for providing the AB signal in accordance with the following equation: AB b(RAB)/PKRH where RAB is the other flow rate of the fresh acid and discharge acid flow rates and RH is the flow rate of the hydrocarbon product.
 6. A system as described in claim 5 in which the G signal means includes means for providing signals corresponding to constants a1 and a2 ; means for providing a signal corresponding to a quantity PR; S signal means for providing signals corresponding to quantities SB, S1 and S2 for the existing hydrocarbon content condition, the increased hydrocarbon content condition and the decreased hydrocarbon content condition, respectively, and means connected to the a1-a2, PR, S, HB, HC1-HC2 signal means for providing the GB, GC1 and GC2 signals in accordance with the a1, a2, PR, SB, S1, S2, HB, HC1 and HC2 signals and the following equation: G a1S2+a2PRH where the GB signal is developed using the SB and HB signals, where SB and HB are substituted for S and H, respectively, the GC1 signal is developed using the S1 and HC1 signals, where S1 and HC1 are substituted for S and H, respectively, and the GC2 signal is developed using the S2 and HC2 signals, where S2 and HC2 are substituted for S and H, respectively.
 7. A system as described in claim 6 in which one of the signals provided by the characteristic sensing means corresponds to the ratio PB of propylene to olefins; and the PR signals means includes means for providing a signal corresponding to an upper limit PU for the ratio of propylene to olefins, means connected to the characteristic sensing means and to the PU signal for comparing the PB and PU signals, means connected to the characteristic signal means and to the PB signal means for providing a signal corresponding to PB/PU, means for providing a signal corresponding to 1.0, and switching means controlled by the comparing means to pass the 1.0 signal as the PR signal while blocking the PB/PU signal when the ratio PB is equal to or greater than the limit ratio PU and to pass the PB/PU signal as the PR signal while blocking the 1.0 signal when the ratio PB is less than the limit ratio PU.
 8. A system as described in claim 7 in which the S signal means includes means for providing a signal corresponding to the volume CV of the contactor, means connected to the characteristic sensing means and to the flow rate sensing means for providing a signal corresponding to the flow rate RO of the olefins in accordance with the characteristic signals and the olefin-isoparaffin flow rate signal, means connected to the CV signal means, to the RO signal means, to the HB signal means, to the flow rate sensing means, and to the HC1-HC2 signal means for providing the SB, S1 and S2 signals in accordance with the RO, CV, HB, HC1 and HC2 signals, the flow rate signals and the following equation: S 100Ro (RR+RH)/CVRR (100-H) where RR and R H are the flow rates of the recycle acid and hydrocarbon product, respectively, the SB, S1 and S2 signals are developed using the HB, HC1 and HC2, respectively, where SB, S1 and S2 are substituted for H in the last mentioned equation.
 9. A system as described in claim 8 further comprising means for providing a signal corresponding to the upper limit HM for the hydrocarbon content; and in which the FH signal means includes means connected to the HB signal means and to HM signal means for comparing the HB signal with the HM signal, means for providing signals corresponding to constants C1 through C5, means for providing a signal corresponding to a reference hydrocarbon content HR for the recycle acid, means connected to the C1-C5 signal means, to the HB signal means, to the HC1-HC2 signal means and to HR signal means for providing signals corresponding to FHB, FHC1 and FHC2 in accordance with the C2 through C5 signals and the HB, HC1, HC2 and HR signals and the following equation: FH'' C2+C3 ((H-HR) /100) + C4((H-HR)/100)2 + C5((H-HR)/100)3 where the F''HB, F''HC1 and F''HC2 signals are derived using the HB, HC1 and HC2 signals where FHB, FHC1 and FHC2 are substituted for H, respectively, in the last mentioned equation, and switching means connected to the last mentioned comparing means, to the C1-C5 signal means and to F''HB, F''HC1 and F''HC2 signal means and controlled by the last mentioned comparing means to provide the C1 signal as the FHB, FHC1 and FHC2 signals while blocking the F''HB, F''HC1 and F''HC2 signals when the existing hydrocarbon content HB is not less than the upper limit HM to the hydrocarbon content and to provide the F''HB, F''HC1 and F''HC2 signals as the FHB, FHC1 and FHC2 signals, respectively, while blocking the C1 signal when the existing hydrocarbon content HB is less than the upper limit HM hydrocarbon content.
 10. A system as described in claim 9 in which the FS signal means includes means for providing signals corresponding to constants d1 through d4, and means connected to the pB signal means, to the d1-d4 signal means and the S signal means for providing the signals corresponding to the quantities FSB, FSC1 and FSC2 in accordance with the SB, S1, S2, d1 through d4 and PB signals and the following equation:
 11. A system as described in claim 1 in which the control signal means is a digital computer programmed in a preDetermined manner to provide the control signals.
 12. A system as described in claim 11 in which the computer has been programmed to determine changes Delta EA1 and Delta EA2 in earnings for an increased hydrocarbon content HC1 for the recycle acid and a decreased hydrocarbon content HC2 for the recycle acid in accordance with the following equations: Delta EA1 ( Delta Q1) (WQ) ( Delta A1) (WA) Delta EA2 ( Delta Q2) (WQ) ( Delta A2) (WA) where Delta Q1 and Delta Q2 are changes in the octane ratings for the increased and decreased hydrocarbon content conditions, respectively, Delta A1 and Delta A2 are changes in the acid consumption for the increased and decreased hydrocarbon content conditions, respectively, and WQ and WA are economic values associated with the octane rating and the acid.
 13. A method for controlling an alkylation unit to achieve a desired hydrocarbon content in recycle acid, said alkylation unit includes a contactor wherein olefins-isoparaffin entering the contactor are contacted in the presence of acid and said contactor provides an acid-hydrocarbon mixture to a settler which separates the acid to provide a hydrocarbon product, including alkylate, and hydrocarbon enriched acid, a portion of the enriched acid is discharged, fresh acid is added to the remaining enriched acid which is then provided to the contactor as recycle acid, which comprises controlling one flow rate of the fresh acid and discharge acid flow rates relative to the other flow rate in accordance with a control signal; sensing characteristics of the olefins-isoparaffin, the fresh acid, the recycle acid and the hydrocarbon product; providing signals corresponding to the sensed characteristics; sensing the flow rates of the olefins-isoparaffin, the recycle acid, the discharge acid and the hydrocarbon product; providing signals corresponding to the sensed flow rates; sensing the interface level in the settler; providing a signal representative of the sensed interface level; providing signals WQ and WA corresponding to economic values related to the octane rating of the alkylate and to the cost of the acid, respectively; and providing the control signal in accordance with the characteristic signals, the flow rate signals, the level signal and the economic signals to control the one acid flow rate so as to control the interface level to achieve the desired hydrocarbon content in the recycle acid and said control signal step includes providing a signal corresponding to the hydrocarbon content HB of the recycle acid in accordance with the characteristic signals; providing signals corresponding to an increased hydrocarbon content HC1 for the recycle acid and for a decreased hydrocarbon content HC2 for the recycle acid; providing signals corresponding to a change in earnings Delta EA1 for an increased hydrocarbon content condition and Delta EA2 for a decreased hydrocarbon content condition in accordance with the HB, HC1 and HC2 signals, selecting the HB, HC1 or HC2 signal in accordance with the Delta EA1 and Delta EA2 signals as a desired hydrocarbon content condition, and providing a signal corresponding to the selected signal as the control signal.
 14. A method as described in claim 13 in which the earnings change signal step includes providing signals corresponding to a change in octane rating Delta Q1 for the increased hydrocarbon condition and to a change in octane rating Delta Q2 for the decreased hydrocarbon condition, providing signals corresponding to a change in acid consumption Delta A1 for the increased hydrocarbon content condition and to a change in acid consumption Delta A2 for the decreased hydrocarbon condition, And providing the Delta EA1 and Delta EA2 signals in accordance with the following equation: Delta EA (Q) (WQ) ( Delta A) (WA) where the Delta EA1 signal is derived by using the Delta Q1 and Delta A1 signals, where Delta Q1 and Delta A1 are substituted for Delta Q and Delta A1, respectively, and the Delta EA2 signal is derived by using the Delta Q2 and Delta A2 signals, where Delta Q2 and Delta A2 are substituted for Delta Q and Delta A, respectively.
 15. A method as described in claim 14 in which the step for providing Delta Q1 and Delta Q2 signals includes providing signals corresponding to a quantity GC1 for the increased hydrocarbon content, to a quantity GC2 for a decreased hydrocarbon content condition and to a quantity GB for the existing hydrocarbon content condition, and providing the Delta Q1 and Delta Q2 signals in accordance with the GC1, GC2 and GB signals and the following equation: Delta Q GC-GB where the Delta Q1 signal is developed using the GC1 and GB signals, where GC1 is substituted for GC and the Delta Q2 signal is developed using the GC2 and GB signals, where GC2 is substituted for GC in the last mentioned equation.
 16. A method as described in claim 15 in which the step of providing the Delta A1 and Delta A2 signals includes providing a signal corresponding to the existing acid consumption AB, providing signals corresponding to the quantities FSB, FSC1 and FSC2 for the existing hydrocarbon content condition, the increased hydrocarbon content condition and the decreased hydrocarbon content condition respectively, providing signals corresponding to the quantities FHB, FHC1 and FHC2 for the existing hydrocarbon content condition, and for the decreased hydrocarbon content condition, respectively, providing the signals corresponding to Delta A1 and to Delta A2 in accordance with the AB, FSC, FHBFSB and FHC signals and the following equation: A AB((FSCFHB/FSBFHC) -1 ) where the Delta A1 signal is provided using the FSC1 and FHC1 signals where FSC1 and FHC1 are substituted for FSC and FHC, respectively, and the Delta A2 signal is developed using the FSC2 and FHC2 signals where FSC2 and GHC2 are substituted for FSC and FHC, respectively, in the last mentioned equation.
 17. A method as described in claim 16 in which the step of providing the AB signal includes providing a signal corresponding to a constant b, providing a signal corresponding to the volume fraction Pk of alkylate in the hydrocarbon product, and the AB signal in accordance with the following equation: AB b(RAB)/PKRH where RAB is the other flow rate of the fresh acid and discharge acid flow rates and RH is the flow rate of the hydrocarbon product.
 18. A method as described in claim 17 in which the step of providing the GC1, GC2 and GB signals includes providing signals corresponding to constants a1 and a2; providing a signal corresponding to a quantity PR; providing signals corresponding to quantities SB, S1 and S2 for the existing hydrocarbon content condition, the increased hydrocarbon content condition and the decreased hydrocarbon content condition, respectively; and providing the GB, G1 and G2 signals in accordance with the a1, a2, PR, SB, S1, S2, HB, HC1 and HC2 signals and the following equation: G a1S2+a2PRH where the GB signal is developed using the SB and HB signals, where SB and HB are substituted for S and H, respectively, the GC1 signal is developed using the S1 and HC1 signals where S1 and HC1 are substituted for S and H, respectively, and the GC2 signal is developed using the S2 and HC2 signals, where S2 and HC2 are substituted for S and H, respectively, in the last mentioned equation.
 19. A method as described in claim 18 in which one of the characteristic signals provided corresponds to the ratio PB of propylene to olefins; and the step of providing the PR signal includes providing a signal corresponding to an upper limit PU for the ratio of propylene to olefins, comparing the PB and PU signals, providing a signal corresponding to PB/PU, providing a signal corresponding to 1.0, and providing the 1.0 signal as the PR signal when the ratio PB is equal to or greater than the limit ratio PU and providing the pB/PU signal as the PR signal when the ratio PB is less that the limit ratio PU.
 20. A method as described in claim 19 in which the step of providing the S signal includes providing a signal corresponding to the volume CV of the contactor, means connected to the characteristic sensing means and to the flow rate sensing means for providing a signal corresponding to the flow rate RO of the olefins in accordance with the characteristic signals and the olefinisoparaffin flow rate signal, providing the SB, S1 and S2 signals in accordance with the RO, CV, HB, HC1 and H2 signals, the flow rate signals and the following equation: S 100RO (RR+RH)/CVRR (100-H) where RR and RH are the flow rates of the recycle acid and hydrocarbon product, respectively, the SB, S1 and S2 signals are developed using the HB, HC1 and HC2 signals, respectively, where HB, HC1 and HC2 are substituted for H in the last mentioned equation.
 21. A method as described in claim 20 further comprising the step of providing a signal corresponding to an upper limit HM for the hydrocarbon content; and in which the step of providing the FH signal includes comparing the HB signal with the HM signal, providing signals corresponding to constants C1 through C5, providing a signal corresponding to a reference hydrocarbon content HR for the recycle acid, providing the FHG, FHC1 and FHC2 signals in accordance with the C2 through C5 signals and the HB, HC1, HC2 and HR signals and the following equation: FH C2 + C3 ((H-HR)/100) + C4((H-HR)/100)2 + C5((H-HR)/100)3 where the FHB1, FHC1 and FHC2 signals are derived using the HB, HC1 and HC2 signals, respectively, where HB, HCl and HC2 are substituted for H in the last mentioned equation, providing the C1 signal as the FHB, FHC1 and FHC2 signals when the existing hydrocarbon content HB is not less than the upper limit HM hydrocarbon content and providing the FHB, FHC1 and FHC2 signals as the FHB, FHC1 and FHC2 signals, respectively, when the existing hydrocarbons content HB is less than the upper limit HM hydrocarbon content.
 22. A method as described in claim 21 in which the step of providing the FS signal includes providing signals corresponding to constants d1 through d4, and providing the signals corresponding to the quantities FSB, FSC1 and FSC2 in accordance with the SB, S1, S2, d1 through d4 and PB signals and the following equation: 